Vertical distribution of seismic forces 1

[jdcivil2012]

I have a brief question regarding the interpretation of section 12.8.3 (ASCE-7 05). Kindly see the attached file. This is a power control room elevation, that has heavy equipments (130 kips) anchored to the floor beams. Now, since this structure has an occupancy category-III, I cannot use the simplified design for the vertical distribution of seismic forces. My question to you is, if I reduce the height of the piers (h.b), most of the total base shear goes to the roof. So theoretically, if I place this control room very close to the ground (say just 1'' above the ground), then almost all of the base shear load will be going to the roof level. Practically this does not look appropriate to me. I think that only the roof dead load + half the wall dead load should be considered for the seismic loads at the roof level & the equipment be considered only at the base of the control room. Is this assessment correct? Or is my interpretation misplaced?

I have attached a very basic calculation to show that the roof seismic load increases substantially with the decrease in the pier height. The CASE-1 shows that if the pier height is too small (which actually is a case on one of my projects), most of the total base shear (67.4%) gets to the roof. And for the CASE-2 only 19.2% gets to the roof. It seems as if the code has left the CASE-1 for interpretation by the engineers.

Thanking you in anticipation for your reply.

 

[structSU10]

This formula, from my inspection of it, assumes a consistent stiffness of each story for this load determination. Due to this, i think the results become skewed, but I'm not 100% sure of this.

Based on the type of structural irregularity you may have, ASCE may force you to do a modal analysis. See section 12.3 which will refer you to table 12.6-1.

I would think with the short first floor, you would have vertical irregularity type 1, which would result in having to do a modal analysis, or time history analysis.

In this case a modal analysis / time history analysis may be appropriate anyway. The ASCE equations are approximations on the vertical distribution of the seismic force. A modal analysis will combine for loads from the response spectrum analysis based on SRSS or CQC, which may give you a better picture of the correct distribution.